The present invention relates generally to semiconductor packaging, and, more specifically, to a Quad Flat Non-leaded (QFN) package with a wettable flank.
Semiconductor integrated circuits are continually decreasing in size and there is a corresponding demand for smaller yet denser circuits. At the same time, there is a desire for such circuits to provide the same or more inputs and outputs. Some types of semiconductor packages have a housing from which leads or lead fingers protrude. The lead fingers are used to interconnect the semiconductor package to external circuitry. Such packages may have a large footprint and the lead fingers increase package height.
As an alternative to packages with protruding lead fingers, a Quad Flat Non-leaded (QFN) package has been developed, which includes exposed contact pads or terminals underneath and on four sides of a rectangular semiconductor package formed with a semiconductor die mounted to a lead frame. The lead frame is formed from a sheet of metal that comprises a die attach pad often called a flag and arms (tie bars) that attach the flag to a frame. Down-set leads of the lead frame are electrically connected to electrodes of the die with bond wires. After wire bonding, the semiconductor die and pads are encapsulated in a plastic compound (material) leaving only underside sections of the leads exposed. The encapsulated semiconductor die and pads form a partially completed package integral with the sheet. The partially completed package is then cut (often referred to as a singulation process) from the sheet to form a rectangular package, where the underside sections of the leads provide contact pads adjacent the four sides of the package.
The lead frame typically comprises copper that is pre-plated with a material like Palladium, which prevents the copper from oxidizing. However, when the devices are singulated, the sides of the sides of the leads are cut, which removes the Palladium pre-plating. This leaves the ends of the leads subject to oxidation. It also makes them more difficult to solder when mounting the device to a printed circuit board. It would be advantageous to have a QFN package that allows for improved quality of the solder joint connections.